I'm looking for an application that can help manage a library of pdf documents. I prefer open source (or just free beer), but that's not absolutely essential.

I am aware of calibre and use it often for my ebooks, including those in pdf format. However, I'd like something for more general purpose use, especially PDFs that are largely - or entirely - graphics based, or contain small amounts of text (like keyboard shortcut reference cards and the like, research papers, and the like).

Easy management of metadata, search and filter, and easy access to the files from an application interface are the other main requirements. I don't need any device-sync or transfer capabilities, all the documents in this library would reside only on my MBP.

DEVONthink
After a hundred years of research I decided on DEVONthink Pro Office for my academic workflow.

I recommend that you don't simply compare the features of the software listed here, but google around for how other people in your field use these apps. It will give you a better idea of what you actually want without trying them all.